[scheduler] Group all task information from PostTask into PostedTask.

Group all information passed to PostDelayedTask (callback, posted from
and delay) into a struct to plumb it together to a place inside
scheduler where a sequence number is generated and PendingTask is
created.

This will allow for easier plumbing for additional task metadata that
we might want to add in the future.

R=alexclarke@chromium.org
CC=skyostil@chromium.org

Bug: 
Change-Id: Iac00cd6c8ce5ac468116ee8bce17894da2d9bc79
Reviewed-on: https://chromium-review.googlesource.com/664703
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501632}
4 files changed